Saturday science subject: Giant black holes

— 10:30 AM on September 6, 2008

Astronomers these days tend to agree that gigantic, supermassive black holes lurk at the center of most galaxies. But how big can those get? The New Scientist says two astronomers—Priya Natarajan of the Harvard-Smithsonian Center for Astrophysics and Ezequiel Treister of the European Southern Observatory—have devised a technique to determine just that.

In a study to appear in Monthly Notices of the Royal Astronomical Society, the pair examined the "feeding habits" and growth of black holes. They used data from surveys carried out by other teams that observed the X-rays and visible light emitted by matter as it is devoured by black holes. The properties of this radiation can be used to estimate a black hole’s mass and how quickly it is gobbling up its surroundings.

The team analysed how many galactic black holes of various masses were present at each stage in the universe's history. The distribution of masses they found today and in the past can only be explained if there is a limit on how fast black holes can grow, the researchers say.

Using their method, Natarajan and Treister worked out that the biggest black holes in the early universe were up to 18 billion times more massive than our Sun. By now, those same black holes "may have bloated to 'ultramassive' size . . . with between 5 and 50 billion times the sun's mass, at the most." For reference, the Sun is roughly 330,000 times more massive than the Earth, and the New Scientist says a black hole at the lower end of the 5-50 billion range would already be "more than 3 times as wide as our solar system." (That figure presumably refers to the black hole's event horizon.)

Such "ultramassive" black holes may just be lying out there, waiting to be found. The New Scientist goes on to say galaxy OJ 287, which sits 3.5 billion light years away, might play host to a pair of orbiting black holes at its center—and radiation outbursts suggest the biggest of the two has 18 billion solar masses. That said, according to Scott Tremaine of the Institute for Advanced Study in New Jersey, using radiation to measure black-hole mass "is fraught with uncertainty because a black hole's brightness can vary depending on how much material it eats."

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